1. Field of the Invention
The present invention relates to EL display apparatus and drive method of EL display apparatus which employs a self-luminous display panel (display apparatus) such as an EL display panel (display apparatus) using organic or inorganic electroluminescent (EL) elements, or the like.
2. Related Art of the Invention
With active-matrix image display apparatus which employ an organic electroluminescent (EL) material or an inorganic EL material as an electrochemical substance, emission brightness changes according to current written into pixels. An EL display panel is of a self-luminous type in which each pixel has a light-emitting element. EL display panels have the advantages of being more viewable than liquid crystal display panels, having high light emission efficiency, requiring no backlighting, having high response speed, etc.
Such an organic EL display panel of an active-matrix type is disclosed in Japanese Patent Laid-Open No. 8-234683.
An equivalent circuit for one pixel of the display panel is shown in FIG. 2. A pixel 16 consists of an EL element 15 which is a light-emitting element, a first transistor (driving transistor) 11a, a second transistor (switching transistor) 11b, and a storage capacitance (capacitor) 19. The light-emitting element 15 is an organic electroluminescent (EL) element. According to this specification, the transistor 11a which supplies (controls) current to the EL element 15 is referred to as a driver transistor 11. A transistor, such as the transistor 11b shown in FIG. 2, which operates as a switch is referred to as a switching transistor 11.
The operation shown in FIG. 2 is described below. A video signal of voltage which represents brightness information is applied to the source signal line 18 with the gate signal line 17 selected. According to the selection by the gate signal line 17, the transistor 11a conducts (wherein the transistor 11a is closed: turned on) and the video signal is charged to the storage capacitance 19. When the gate signal line 17 is deselected, the transistor 11a is open (turned off). The transistor 11b is cut off electrically from the source signal line 18. However, the gate terminal potential of the transistor 11a is maintained by the storage capacitance (capacitor) 19. Current delivered to the light-emitting element 15 via the transistor 11a depends on gate-drain terminal voltage Vgd of the transistor 11a. The light-emitting element 15 continues to emit light at an intensity which corresponds to the amount of current supplied via the transistor 11a. 
A driver circuit which drives a pixel configuration in FIG. 2 outputs a voltage video signal. The driver circuit which outputs the voltage video signal has a configuration similar to that of a driver circuit which drives a liquid crystal display panel. The driver circuit applies a voltage signal as the video signal to the source signal line 18. The applied voltage signal is applied to the pixel 16 and held in a capacitor 19.
However, an organic EL display panel uses transistor arrays made of low-temperature or high-temperature polysilicon, and variations in the characteristics of the transistors in the polysilicon transistor arrays of organic EL elements will cause display irregularities.
FIG. 2 shows a pixel configuration for voltage programming mode. The voltage programming mode involves a configuration, circuit, or drive method which applies a video signal or other voltage signal (program voltage) given as a magnitude or intensity of a voltage to a data signal line, source signal line, or pixel, converts the voltage signal into a current signal using pixel transistors, and applies the resulting current signal to the EL element.
Current programming mode involves applying a video signal or other current signal (program current) given as a magnitude or intensity of a current to a data signal line, source signal line, or pixel, and applies the resulting current signal using pixel transistors to the EL elements.
Both an act of causing a current to flow into an EL element 15 from a driving transistor 11 and act of causing a current to flow into a driving transistor from an EL element 15 are referred to as applying the current from a driving transistor 11 to an EL element 15. In other words, the current programming mode involves a configuration, circuit configuration, or drive method which applies a current signal (program current) that is approximately proportional to the applied current or obtained by converting the applied current in a predetermined manner, either directly or indirectly to the EL element.
With the pixel configuration illustrated in FIG. 2, a voltage video signal is converted into a current signal by the transistor 11a. Thus, any variation in the characteristics of the driving transistor 11a will cause variations in the resulting current signal. Generally, the driving transistors 11a have 50% or more variations in their characteristics. Consequently, the configuration in FIG. 2 causes display irregularities accordingly.
The voltage programming mode has a low capability to compensate for variations in transistor characteristics of the pixel 16. Thus, it involves display irregularities resulting from the variations in transistor characteristics. However, the voltage programming mode has a high capability to charge and discharge source signal lines and the like both in low tone and high tone regions. Thus, it can achieve proper image display without causing insufficient writing.
The display irregularities can be reduced by using the configuration of the current programming mode. The current programming mode provides low drive current in low tone regions. Consequently, parasitic capacitance of the source signal line 18 can prevent proper driving.
Incidentally, current programming (mode) is also called current driving and voltage programming (mode) is also called voltage driving.
To solve the conventional problem described above, the present invention has an object to provide an EL display apparatus and a drive method thereof which can eliminate insufficient writing in all tone regions while reducing display irregularities.